1. Field of the Invention
The present invention relates to a method of cooling steel plate applied in order to enable uniform top and bottom cooling in a case of spraying a coolant (cooling medium comprised of water or a mixture of water and air, hereinafter referred to as “cooling water”, “coolant”, and “water”) on the top and bottom surfaces of steel plate (mainly thick-gauge steel plate, hereinafter referred to as “steel plate”) with a temperature of several hundreds of degrees or more when constrained and conveyed between a plurality of pairs of constraining rolls in a hot rolling process or a heat treatment process of the steel plate so as to thereby obtain steel plate having uniform shape characteristics and material characteristics and a high quality.
2. Description of the Related Art
For example, a steel plate production facility provided with a process referred to as “controlled cooling” which rapidly cools (acceleratedly cools) high temperature steel plate right after hot rolling by cooling water to obtain a quenching effect and impart high strength characteristics to the steel plate is in practical use.
As the controlled cooling apparatus used here, Japanese Patent Publication (A) No. 61-1420, FIG. 1, etc. discloses the technology of arranging header mechanisms provided with pluralities of nozzles at the top and bottom surface sides of steel plate after hot rolling by a hot finishing mill and spraying cooling water from the groups of top and bottom nozzles to forcibly cool the steel plate.
However, in such a conventional steel plate production facility provided with such a controlled cooling apparatus, there is the problem that when acceleratedly cooling steel plate by the controlled cooling apparatus, shape defects occur due to warping more easily than the conventional case using air cooling due to the unbalance of cooling etc. of the top and bottom surfaces of the steel plate.
These shape defects are mainly caused by the difference of cooling rates due to the difference of behaviors of cooling water sprayed from the top surface side and the bottom surface side of the steel plate or the difference of flows of the cooling water in the plate width direction. Asymmetric internal stresses are generated in the plate thickness direction and the plate width direction causing deterioration of the shape of the product. In remarkable cases, in addition to these shape defects, there sometimes arises the problem of a drop in the mechanical properties such as the strength and elongation of the steel material.
Further, there is also the problem of easy occurrence of variations in quality among products when producing a large number of products having the same specifications. This is mainly due to variations in transformation of the steel material structure due to fluctuations in the cooling stop temperature.
In recent years, tougher restrictions have been placed on uniformity of mechanical properties of steel plate and on variations in production lots when producing products having the same specifications.
At the present, in order to allow variation at the time of cooling and maintain the products at a constant quality or more, variation of the cooling stop temperature is being compensated for by the control of steel ingredients, rolling pattern, etc., by reheat treatment after production, etc. If the variation of the cooling stop temperature is reduced, the economical effects enjoyed become very large, for example, production conditions such as the steel ingredients and rolling pattern can be eased and the heat treatment after production can be omitted.
Further, as technology preventing variation of the cooling stop temperature at the time of cooling the top and bottom surfaces of steel plate to prevent occurrence of shape defects and realizing stability of mechanical properties, conventionally there has been the technology of measuring the temperatures at the top and bottom surfaces of the steel plate at the time of water cooling, predicting the amount of deformation from the temperature difference, and controlling the amounts of water sprayed to the top and bottom surfaces of the steel plate so as to prevent the deformation.
For example, as described in the claim of Japanese Patent Publication (A) No. 2-179819, there is disclosed a cooling control apparatus of hot rolled steel plate having the functions of securing a cooling end temperature previously determined based on the quality of the material and controlling the amounts of cooling water sprayed from the top and bottom surfaces so that the amount of warping of the hot steel plate at the time of the water cooling falls within a prescribed value.
In the technology disclosed in Japanese Patent Publication (A) No. 2-179819, the relationship between the amounts of cooling water and heat transfer coefficients is found in units of the top surface and bottom surface based on various physical properties of the hot steel plate given in advance, the temperature histories in the cooling process in the distribution of temperature in the plate thickness direction are predicted from this relationship, the amount of warping of the hot steel plate is predicted from the temperature distribution histories, and the amounts of cooling water sprayed from the top and bottom surfaces are controlled so that this amount of warping falls within the prescribed range.
In this technology, a cooling zone is formed using the spaces in the conveyance direction between a plurality of pairs of constraining rolls as the control units. In this cooling zone, the amounts of cooling water of the groups of top surface nozzles and the groups of bottom surface nozzles between the pairs of constraining rolls are controlled to the same amounts. A plurality of these cooling zones are arranged to enable adjustment (selective use) of the cooling zones used according to the plate thickness, plate length, and other conditions and the cooling start temperature, cooling stop temperature, and other factors. Then, it is disclosed to control the cooling of the steel plate by changing the amounts of the sprayed water and the conveyance speed. Further, it is disclosed to correct the cooling rate, which differs between mask portions at the end portions and a center portion, in the width direction of the hot steel plate. At this time, as the predicted value of the heat transfer coefficient at the time of cooling used for computation of the temperature histories, the heat transfer coefficient, which changes due to the amounts of the sprayed water and the steel plate temperature as factors, is set in each cooling zone described above.
However, in the technology of Japanese Patent Publication (A) No. 2-179819, for example, as shown FIG. 10, when cooling steel plate 1 being constrained and conveyed between the pairs of constraining rolls 21 and 22 in a steel plate cooling region (distance L: about 0.7 m to 1.5 m in usual cases) of a cooling apparatus 6 provided with groups of top and bottom surface nozzles 6a and 6b each having pluralities of nozzles 3, it is difficult to stably secure precision of cooling control and it is difficult to sufficiently respond to the above-described demands.
According to discoveries by the present inventors, in order to predict the temperature histories of steel plate with a good precision and control the amounts of sprayed coolant in accordance with the prediction with a high precision, it is necessary to sufficiently consider the transition of the heat transfer coefficient as it changes in the steel plate conveyance direction and the steel plate width direction in the steel plate cooling region between pairs of constraining rolls.
However, in the technology of Japanese Patent Publication (A) No. 2-179819, this is not sufficiently considered, therefore the precision of prediction of the heat transfer coefficient becomes insufficient. This is particularly remarkable when changing the conveyance speed in the steel plate conveyance direction.
Accordingly, in the technology of Japanese Patent Publication (A) No. 2-179819, in order to further reduce the difference in temperature histories between the top and bottom surfaces of the steel plate, stably secure the shape characteristics and mechanical characteristics, and secure steel plates able to sufficiently respond to the increasing severe demands for quality, further reinforcement of the cooling control conditions is demanded.